From your experiences with alarm systems, how loud can a siren be in a one floor house and can it be heard all over as though it were close to you? Would a house with two floors need a sounder upstairs? I personally never heard an alarm go off, though I my family had an alarm when I was a kid.

Probably depends on the system.
I have found the ones in my house can be considered very damn loud. We were on vacation and the system triggered. The neighbors 2 houses down heard it. They were the ones house sitting for us.
Btw… The alarm triggered because we had a bottle of champagne before we left and it was on the kitchen table. The dogs knocked it off the table and the glass breakage alert went off. No harm done.
dont know